Chimpanzees

A female's estrus cycle averages 38 days, which includes 2-4 days of menstruation. When a female begins her estrus cycle, her genital area swells for approximately ten days, and this is when she is sexually attractive and receptive. The last three or four days of estrus is when the likelihood of conception is highest. Mating is seemingly random and varied, and receptive females are often mounted by most of the mature males in the community. A high-ranking male may, however, claim "possession" and prevent other males from mating with a female. Regardless of rank and social status in the community, all males and females have a chance to pass on their genes.

On average, female chimpanzees give birth every five to six years. Gestation is 230-240 days. Newborn chimps have only a weak grasping reflex, and initially require full support from their mother as she moves about. After a few days, however, the infant is able to cling securely to its mother's underside. About the age of five to seven months, the youngster is able to ride on its mother's back. At the age of four years, a young chimp can travel well by walking. Weaning occurs before its third year, but the youngster will stay with its mother until it is five to seven years old.

When an infant chimp is born, its older sibling will start to become more independent of its mother. It will look for food and will build its own sleeping nest, but a close relationship remains with the mother and develops between the siblings. Young males stay close to their family unit until about the age of nine. At this time, they find an adult male to follow and watch his behavior. Thus begins the long process by which the male develops his place in the community.

Young females stay with their mothers until about ten years of age. After her first estrus, a young female typically withdraws from her natal group and moves to a neighboring one, mating with its males. At this time a female may transfer out of her initial group to form a family of her own. This exchange helps to prevent inbreeding and maintains the diversity of the gene pool.